ABSTRACT In thermoelectric industry, synthetic gypsum and coal ash are generated as industrial by-products. The objective of this work was to evaluate the effect of different mixtures of synthetic gypsum and coal ash on N mineralization in an Alfisol. Nitrogen mineralization was evaluated by soil incubations under controlled conditions and potential mineralization using the nonlinear first order kinetics model Nmt = N0 [1 - &#8494;(-k t )] , where Nmt is the mineral N accumulated at a specific time, N0 is the potentially mineralizable N, k is the mineralization rate, and finally t is the incubation time. Treatments consisted of different mixtures of synthetic gypsum, coal ash and urea (M1: 50% synthetic gypsum, 50% coal ash, M2: 50% synthetic gypsum, 35% coal ash, 15% urea, M3: 45% synthetic gypsum; 40% coal ash, 15% urea, M4: 65% synthetic gypsum, 20% coal ash, 15% urea, M5: 55% synthetic gypsum, 30% coal ash, 15% urea). Each mixture was applied to the soil at 2 t ha-1. The results shown the highest concentration (p < 0.05) of net mineral N (NMN) applying M3 (166.14 mg kg-1), while the lowest NMN (p < 0.05) was found applying M5 (98.30 mg kg-1). Treatment M1 presented the highest N0 value (579.0 mg kg-1) respect to treatments with urea in its composition, with exception of M3 (523.4 mg kg-1). Where the highest and lowest k were determined in M5 (k = 0.44990) and M3 (k = 0.04470), respectively. Mixtures of synthetic gypsum, coal ash and urea increased N mineralization over mixtures without urea. Respect N potential mineralization, in some cases N0 and k values would respond more to the fit of the mathematical model employed than to the biological process.